Chemistry Reference
In-Depth Information
0.45 V, associated with a cathodic peak E
p
at the much lower potential
cyclic voltammetry curve at
+
0.03 V. E
p
is found to be higher than E
1
+
values
of the corresponding mononuclear complex obtained
/
2
in pyridine [Cu
II
(L
OMe2
4
)(py)]
+
00 V). This is not surprising as coordination to two instead
of one positively charged metal ion makes the oxygen less electronegative. In contrast, E
p
is close
to E
1
/
2
(E
1
/
2
=
0
.
of [Cu
II
(L
OMe2
4
)(py)]
+
, thus showing that [Cu
II
2
(L
OMe2
4
)
2
] is likely converted into a monomer
upon oxidation. As bulk electrolysis of [Cu
II
2
(L
4
OMe,tBu
)
2
] produces species that are characterized by
≈
strong absorption bands at
410 and 600 nm, it has been proposed that the monomer is the radical
[Cu
II
(L
OMe2
4
)
•
(CH
3
CN)]
2
+
.
23
The dimers formed with N
2
O
2
ligands are very interesting as the ligand exhibits two distinct phe-
nolates, one bridging and one (presumably easier to oxidize) non-bridging. The radical species remain
dimeric in this case as the phenoxyl is generated on the non-bridging phenolate. The protypical complex
[Cu
II
2
(L
6
OMe
)
••
2
]
2
+
(Figure 8.17) exhibits the expected phenoxyl band at 416 nm (3370 M
−
1
cm
−
1
and is
found to be EPR silent as the result of antiferromagnetic exchange coupling between the two copper ions
(separated by 2.8 - 3 A) mediated by the
)
-phenolato bridges. It is noteworthy that the stability of these
dimeric radicals is low compared to the monomers. For instance the half-life of [Cu
II
2
(L
6
OMe
µ
)
••
2
]
2
+
is five
minutes at 298 K, that is much lower than that of the monomer [Cu(L
4
OMe
)
•
(Py)]
2
+
(154 minutes at 298 K)
although the former possesses a similar 2-
tert
-butyl-4-methoxyphenoxyl moiety.
22
The sole exception is
[Cu
II
2
(
)
••
2
]
2
+
, which was found to be exceptionally stable.
21
In some cases, dissolution of the
dimers in coordinating solvents results in the formation of monomers, as observed for [Cu
II
2
(L
6
H
im
L
6
H
)
2
]. The
)
•
(CH
3
CN)]
+
characterized by an unusual broad
oxidation product is then the radical monomer [Cu(L
6
H
≈
EPRsignalatg
2. This signal likely arises from a weak ferromagnetic or a dipole - dipole interaction
between the copper(II) and the phenoxyl radical, suggesting an axial positioning of the phenoxyl radical.
20
[Cu
II
2
(L
7
tBu
)
2
] was also shown to dissociate upon oxidation into the radical monomer [Cu
II
(L
7
tBu
)
•
(H
2
O)]
+
that quickly decomposes.
112
There are only two examples of dicopper(II) complexes in which the bridging atom is a phenoxyl
oxygen (only one being stable enough to be characterized).
110
Both examples were isolated from the
dinucleating ligand H
di
L
4
OMe
, depending on the amount of triethylamine added during the synthesis.
[Cu
II
2
(
(CH
3
CN)
2
]
3
+
, which is the precursor of the most stable radical species, contains two
metal ions that are separated by 4.06 A (an exogenous acetonitrile completes to five the metal coordi-
nation sphere). The radical [Cu
II
2
(
di
L
4
OMe
)
di
L
4
OMe
)
•
(CH
3
CN)
2
]
4
+
has a (S
t
=
3/2) ground state, with a D value
0.056 cm
−
1
of
−
intermediate between that of triradicals and that of copper(II) coordinated bis-phenoxyl
radicals.
Radical
complexes
of
salen
ligands
were
first
described
in
1996.
The
phenolate
moieties
of
[Cu
II
na
L
8
SPr
(
)
]are
ortho
substituted by the electron donating SCH
3
group, thus mimicking the thioether
bond in GO.
113
0.62 V results in the formation of [Cu
II
na
L
8
SPr
)
•
]
+
. Although the expected
Oxidation at
+
(
π
* transitions of the phenoxyl radicals were obscured by intense charge transfer transitions, the
copper(II) - radical rather copper(III) - phenolate redox state of the one-electron oxidized complex was
unambiguously established by EXAFS and X-ray absorption spectroscopy. [Cu
II
-
π
)
•
]
+
is EPR
silent as the result of antiferromagnetic coupling between the coordinated radical and the metal spins.
Interestingly, this complex exhibits a significant GO-like catalytic activity, with 1300 turnovers in 20
hours for the aerobic oxidation of benzyl alcohol to benzaldehyde (at 295 K).
114
This may be explained
by the significant tetrahedral distortion induced around the copper atom by the binaphthyl linker, thus
stabilizing the reduced copper(I) form of the catalyst. Other salen complexes were synthesized in order
to modulate the flexibility of the linker, and the geometry around the metal ion. These complexes
usually exhibit two reversible oxidation waves on their cyclic voltammetry curves that are attributed to
the successive formation of mono- and diradical species (Equation 8.17).
115 - 117
Interestingly, the rigid
scaffold of [Cu
II
(L
9
)] enhances the electronic communication between the two phenolate moieties. As a
na
L
8
SPr
(
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