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

2000-06-23T00:00:00Z (GMT) by Sonha C. Payne Karl S. Hagen
The reactions of [Fe<sub>2</sub>(OH)(R<sub>3</sub>CCOO)<sub>2</sub>(Me<sub>3</sub>tacn)<sub>2</sub>]<sup>+</sup> (R = F (<b>1</b>-F), Me (<b>1</b>-Me), Ph (<b>1</b>-Ph)) with dioxygen initially afford a μ-oxo bridged mixed valent diiron(II)(III) complex [Fe<sub>2</sub>O(R<sub>3</sub>CCOO)<sub>2</sub>(Me<sub>3</sub>tacn)<sub>2</sub>]<sup>+</sup> (R = Me (<b>2</b>-Me) and Ph (<b>2</b>−Ph)) and subsequently the diiron(III) complexes <b>3</b>-Me and <b>3-</b>Ph. The reactions in acetonitrile are fast for <b>1</b>-F, slower for <b>1</b>-Me, and slow enough for <b>1</b>-Ph so that crystals of <b>2</b>-Ph as the triflate (<b>2</b>-Ph-OTf) and BPh<sub>4</sub> salts could be isolated and their structures determined. The oxidant <i>N</i>-methylmorpholine <i>N</i>-oxide (MMNO), is cleaner in that the byproduct, H<sub>2</sub>O, does not further oxidize <b>2</b> as does the byproduct, H<sub>2</sub>O<sub>2</sub>, from the reaction with O<sub>2</sub>. The crystal structures indicate that <b>2</b> has an oxo bridge with the Fe···Fe distances in <b>2</b>-Ph-OTf (3.123(1) Å) and <b>2</b>-Ph-BPh<sub>4</sub> (3.155(1) Å) similar to those of <b>3</b>. The magnetic susceptibility data are fit for <b>1</b>-Ph to two equivalent high-spin Fe(II) sites with <i>H</i> = −2<i>J</i><i>S</i><sub>1</sub>·<i>S</i><sub>2</sub>, <i>D</i> = 2.823, <i>g</i> = 2.00, and <i>J</i> = −12.1 cm<sup>-1</sup>, for <b>2</b>-Ph to an <i>S</i> = <sup>1</sup>/<sub>2</sub> ground state with a high-spin Fe(III) site with <i>D</i> = 1.46, <i>g</i> = 2.00, and a high-spin Fe(II) site with <i>D</i> = 13.93, <i>E</i>/<i>D</i> = 0.285, <i>g</i> = 2.069, and <i>J</i> = −144 cm<sup>-1</sup>, and for <b>2</b>-Me to an <i>S</i> = <sup>1</sup>/<sub>2</sub> ground state with a high-spin Fe(III) site with <i>D</i> = 2.39, <i>g</i> = 2.00, and a high-spin Fe(II) site with <i>D</i> = −11.42, <i>g</i> = 2.041 and <i>J</i> = −119 cm<sup>-1</sup>. The EPR spectrum is consistent with the <i>S</i> = <sup>1</sup>/<sub>2</sub> ground state (<i>g</i> = 1.97, 1.93, 1.90 at 10 K) and is observed even at 85 K. The Mössbauer of <b>2</b>-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, Δ<i>E</i><sub>Q</sub> = 2.45, 2.35, and Γ = 0.55, 0.75 mm s<sup>-1</sup>, respectively). Electrospray mass spectra in MeCN with <sup>18</sup>O labeled <b>1</b>-−Ph and <b>1</b>-Me indicated that they reacted by an outer-sphere process with <sup>16</sup>O<sub>2</sub> or MMN<sup>16</sup>O as none of the oxidant is incorporated into <b>2</b>.