Steric control of reactivity of non-heme mu-hydroxo diiron(II) complexes with oxygen: Isolation of a strongly coupled mu-oxo Fe(II)Fe(III) dimer

The reactions of [Fe-2(OH)(R3CCOO)(2)(Me(3)tacn)(2)](+) (R = F (1-F), Me (1 -Me), Ph (1-Ph)) with dioxygen initially afford a mu-oxo bridged mixed vale nt: diiron(IL)(III) complex [Fe2O(R3CCOO)(2)(Me(3)tacn)(2)](+) (R = Me (2-M e) 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 BPh 4 salts could be isolated and their structures determined. The oxidant N-me thylmorpholine N-oxide (MMNO), is cleaner in that the byproduct, H2O, does not further oxidize 2 as does the byproduct, H2O2, from the reaction with O -2. The crystal structures indicate that 2 has an oxo bridge with the Fe .. . Fe distances in 2-Ph-OTf (3.123(1) Angstrom) and 2-Ph-BPh4 (3.155(1) Angs trom) similar to those of 3. The magnetic susceptibility data are fit for 1 -Ph to two equivalent high-spin Fe(II) sites with H = -2JS(1). S-2, D = 2.8 23, g = 2.00, and J = -12.1 cm(-1), for 2-Ph to an S = 1/2 ground state wit h a high-spin Fe(III) sire 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 Mossbauer of 2-Ph exhibits a poorly resolved doublet suggesting a Class IZ mixed valent species (Fe(II) and Fe(III) sites with delta = 1.09, 0.6, Delta E-Q 2.45, 2.35, and Gamma = 0.55, 0.75 mm s(-1), respectively). Electrospray mass spe ctra in MeCN with O-18 labeled 1--Ph and 1-Me indicated that they reacted b y an outer-sphere process with O-16(2) or (MMNO)-O-16 as none of the oxidan t is incorporated into 2.