Chemistry Reference
In-Depth Information
6.3.1.2 Iron(IV)- and Iron(V)-Oxo Complexes.
The redox properties of
nonheme Fe
IV
=O complexes have been examined [279, 328-332]. The Fe
IV
=O
species generally undergoes a H-atom abstracting ability in the order
Fe
IV
=O >> Fe
IV
-O-Fe > Fe
IV
-OH [328]. Among the studied Fe
IV
=O complexes,
[Fe
IV
(O)(N4Py)]
2+
(N4Oy =
N
,
N
-bis(2-pyridylmethyl)-bis(2-pyridyl)methyl-
amine) had the ability to oxidize C-H bonds, even in cyclohexane (D
C-H
=
99.3 kcal/mol) [333]. The [Fe
IV
(O)(N4Py)]
2+
has been synthesized using chemi-
cal, electrochemical, and photocatalytical methods [334-336]. The theoretical
study supports the involvement of proton-coupled electron transfer in the
O-O cleavage to result in oxoiron(IV) from the reaction of nonheme iron(II)
complex with H
2
O
2
[337].
Recently, the rate constants for the oxidation of dihydroanthracene (DHA)
by [Fe
IV
(O)(N4Py)]
2+
(
k
2
, M/s) as a function of D
O-H
were compared with
other oxo complexes of Fe(IV), Ru(IV), and Mn to understand the transfer
of a proton associated with the one-electron transfer process, responsible for
the conversion of the Fe
IV
=O unit to the Fe
III
-OH species (Fig. 6.28) [329,
338-342]. The straight line in the plot of Figure 6.28 was constructed using
appropriate values for
t
BuO
•
and
t
BuOO
•
. This approach was used previously
8
t
BuO
•
4
Ru
IV
1 (25°C)
t
BuOO
•
1 (-15°C)
MnO
4
-
0
Fe
IV
O(TMP) (-15°C)
Mn(O)(OH)
-4
Mn(OH)
2
Mn
2
O
2
Mn
2
(O)(OH)
-8
65
75
85
95
105
D
O-H
(kcal/mol)
Figure 6.28.
Plot of log
k
2
of DHA oxidation and the strength of the O
-
H bond formed
by the oxidants in CH
3
CN at 25°C unless labeled otherwise. Data points shown in black
filled squares were taken from References 32-37 and the blue filled circle was obtained
in the present work of complex
1
. The red straight line was drawn through the points
belonging to the two oxygen radicals following Mayer's precedent. Mn(O)(OH),
[Mn
IV
(Me2EBC)(O)(OH)]
+
; Mn(OH)
2
, [Mn
IV
(Me
2
EBC)(OH)
2
]
2+
; Mn
2
O
2
, [(phen)
2
Mn
IV
(O)
2
Mn
III
-(phen)
2
]
3+
; Mn
2
(O)(OH), [(phen)
2
Mn
III
(O)(OH)Mn
III
(phen)
2
]
3+
; Ru
IV
O,
[Ru
IV
(O)(bpy)
2
(py)]
2+
(adapted from Wang et al. [329] with the permission of the
American Chemical Society). See color insert.
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