Chemistry Reference
In-Depth Information
redox waves attributed as depicted below in Equation 8.8, the fully oxidized and reduced forms are rather
unstable due to the lability of the ligands.
[Fe
III
L
14
AP
L
14
ISQ
)
••
2
]
−
[Fe
III
L
14
ISQ
)
•••
3
]
[Fe
III
L
14
ISQ
)
••
2
(
L
14
BQ
]
+
(
)(
(
(
)
[Fe
III
L
14
ISQ
)
•
(
L
14
BQ
)
2
]
2
+
(
(8.8)
[Fe
III
(L
14
ISQ
)
•••
3
]
The
three
o
-iminobenzosemiquinonate
ligands
of
couple
antiferromagnetically
120 cm
−
1
) to the high spin metal center as in [Cr
III
(L
14
ISQ
)
•••
3
] due to geometrical analogies.
62
(J
=−
[Fe
III
(L
14
ISQ
)
•••
3
] also exhibits the typical bands for the
o
-iminobenzosemiquinonate radical at 423 nm
(6000) and 746 nm (8400 M
−
1
cm
−
1
.[Fe
III
(L
14
ISQ
)
•
(cyclam)]
2
+
75
as well as [Fe
III
(L
14
ISQ
)
•
(tren)]
2
+
76
)
are
diamagnetic due to the antiferromagnetic coupling between the (S
Fe
=
1
/
)
metal spin and the (S
rad
=
1
/
)
2
2
radical spin. These complexes exhibit the typical bands for the
o
-iminobenzosemiquinonate radical at
≈
420
and 650 nm with somewhat weaker intensities compared to [Fe
III
(L
14
ISQ
)
•••
3
] because of the decreased
number of
o
-iminobenzosemiquinonate subunits. It can be noticed that the
o
-iminobenzosemiquinonate
moiety
)
•
(tren)]
2
+
could be either oxidized into a
benzoquinone or reduced into an aminophenolate affording species with closed shell ligands that are
consequently paramagnetic (S
t
)
•
(cyclam)]
2
+
[Fe
III
(L
14
ISQ
[Fe
III
(L
14
ISQ
of
and
=
=
/
)
S
Fe
1
.
2
)
••
2
(X)] the iron atom exhibits a square pyramidal geometry, where
the axial coordination site is occupied by an exogenous anion (halogen X, SCN
−
,N
3
−
)
In the diradical complex [Fe
III
(L
14
ISQ
.
77-79
This fifth
ligand is of prime importance in the stabilization of a particular metal spin state rather than another.
When X
SCN
−
or Cl
−
=
the iron(III) ion is high spin (S
Fe
=
/
2
), and consequently the total spin state
5
is (S
t
=
/
2
) (antiferromagnetic coupling with the two
o
-iminobenzosemiquinonate radicals). When X
=
3
Br
−
,I
−
or N
3
−
the iron can exist either at the low (S
Fe
=
/
2
)
or intermediate (S
Fe
=
/
2
) spin state in a
1
3
temperature-dependent spin crossover behavior. It is noticeable that a (S
t
=
1
/
2
)
ground state is attained in
both cases. The EPR spectrum of [Fe
III
(L
14
ISQ
)
••
2
(I)] is characterized by signals at g
=
2.189 and 2.134
g
e
, confirming that the unpaired electron resides in a d orbital, thus (S
Fe
=
3
/
2
).
In the diradical complex [Fe
III
qui
L
14
ISQ
)
••
2
]
+
the iron is octahedral and low spin.
67
(
Remarkably, despite
, that is similar to that of [Fe
III
(L
14
ISQ
)
••
2
(I)], its EPR spectrum differs
that the ground state is (S
t
=
1
/
2
)
)
••
2
]
+
has
a marked organic radical character, which can only be explained by considering that the iron is (S
Fe
=
1.9905. Therefore, the SOMO of [Fe
III
qui
L
14
ISQ
significantly with a rather isotropic signal at g
=
(
1
/
2
)
and not (S
Fe
=
2
).
Homobimetallic complexes were isolated from several kinds of ligands. In the [(tmtacn)Fe
III
3
/
oxm
L
13
tBu
)
3
Fe
III
] complex, the imine nitrogen and phenolic oxygen of the tridentate Schiff base ligand bind one metal
ion, whereas the terminal oxamate oxygen binds another metal (the coordination sphere of this latter is
completed by the tmtacn ligand).
80
Three oxidation waves attributed to the successive oxidation of the
phenolates into phenoxyl radicals were observed but the radicals could not be quantitatively generated.
In [Fe
III
2
(
(
)
•
(Cl)
4
]
−
, the iron atom does not exhibit an octahedral but rather a square pyramidal
geometry in which three coordinating atoms are provided by the bis(tridentate) ligand and two other from
chloride anions.
81
Et
L
16
oxm
=
/
)
signal at g
x
=
.
The EPR spectrum of this complex displays a rhombic (S
t
1
1
998,
2
g
y
=
.
944, and g
z
=
.
825. This ground state is attained via an intramolecular antiferromagnetic coupling
between an
o
-iminobenzosemiquinonate radical and a high spin ferric ion (J of
1
1
100 cm
−
1
), yielding
≈−
a(S
∗
=
2), which in turn couples weakly antiferromagnetically to the second high spin ferric ion (J
≈
10 cm
−
1
.[Fe
III
2
(
mPh
L
16
N
)
3
6
•
] has been obtained by two independent teams from the same ligand.
69,82
−
)
Compared to [Mn
IV
)
••
2
] it is a hexaradical complex with a singlet ground state
resulting from both ferro- and antiferromagnetic exchanges that will not be detailed here.
mPh
L
16
ISQ
)
••
(
mPh
L
16
N,ISQ
(
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