TY - JOUR
T1 - Redox potential and C-H bond cleaving properties of a nonheme Fe IV=O complex in aqueous solution
AU - Wang, Dong
AU - Zhang, Mo
AU - Bühlmann, Philippe
AU - Que, Lawrence
PY - 2010/6/9
Y1 - 2010/6/9
N2 - High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic FeIV=O complexes characterized to date, [Fe IV(O)(N4Py)]2+ (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (DC-H = 99.3 kcal mol-1). However, its redox properties are not yet well understood. In this work, the effect of protons on the redox properties of 1 has been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclic voltammetry of 1 in CH3CN is complicated by coupling of several chemical and redox processes, the FeIV/III couple is reversible in aqueous solution with E1/2 = +0.41 V versus SCE at pH 4 and involves the transfer of one electron and one proton to give the FeIII-OH species. This is in fact the first example of reversible electrochemistry to be observed for this family of nonheme oxoiron (IV) complexes. C-H bond oxidations by 1 have been studied in H2O and found to have reaction rates that depend on the C-H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed a DO-H value of 78(2) kcal mol-1 to be calculated for the FeIII-OH unit derived from 1. Interestingly, although this DO-H value is 6-11 kcal mol -1 lower than those corresponding to oxidants such as [Fe IV(O)(TMP)] (TMP = tetramesitylporphinate), [RuIV(O)(bpy) 2(py)]2+ (bpy = bipyridine, py = pyridine), and the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparable to rates for these other oxidants. This comparison suggests that the nonheme N4Py ligand environment confers a kinetic advantage over the others that enhances the C-H bond cleavage ability of 1.
AB - High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic FeIV=O complexes characterized to date, [Fe IV(O)(N4Py)]2+ (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (DC-H = 99.3 kcal mol-1). However, its redox properties are not yet well understood. In this work, the effect of protons on the redox properties of 1 has been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclic voltammetry of 1 in CH3CN is complicated by coupling of several chemical and redox processes, the FeIV/III couple is reversible in aqueous solution with E1/2 = +0.41 V versus SCE at pH 4 and involves the transfer of one electron and one proton to give the FeIII-OH species. This is in fact the first example of reversible electrochemistry to be observed for this family of nonheme oxoiron (IV) complexes. C-H bond oxidations by 1 have been studied in H2O and found to have reaction rates that depend on the C-H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed a DO-H value of 78(2) kcal mol-1 to be calculated for the FeIII-OH unit derived from 1. Interestingly, although this DO-H value is 6-11 kcal mol -1 lower than those corresponding to oxidants such as [Fe IV(O)(TMP)] (TMP = tetramesitylporphinate), [RuIV(O)(bpy) 2(py)]2+ (bpy = bipyridine, py = pyridine), and the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparable to rates for these other oxidants. This comparison suggests that the nonheme N4Py ligand environment confers a kinetic advantage over the others that enhances the C-H bond cleavage ability of 1.
UR - http://www.scopus.com/inward/record.url?scp=77953109574&partnerID=8YFLogxK
U2 - 10.1021/ja909923w
DO - 10.1021/ja909923w
M3 - Article
C2 - 20476758
AN - SCOPUS:77953109574
SN - 0002-7863
VL - 132
SP - 7638
EP - 7644
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 22
ER -