Chemistry Reference
In-Depth Information
Table 6.5 Parameters (in cm
1
) for the symmetry analysis of the metal ion sites in [Eu
2
(L
i
)
3
]
(
i
¼5-8, 11). Frozen solutions in water/glycerol 90 : 10 (v/v) at 10 K, unless otherwise stated.
5
D
0
!
7
F
0
fwhh
a
5
D
0
!
7
F
1
DE
(A
2
-E)
5
D
0
!
7
F
1
DE
(E-E)
Helicate
Ref.
15
b
[Eu
2
(L5)
3
]
156
28
[40]
19
b
[Eu
2
(L6)
3
]
162
38
[45]
17
b
[Eu
2
(L7)
3
]
161
31
[46]
[Eu
2
(L8)
3
]
15
161
31
[49]
[Eu
2
(L9)
3
]
17
160
31
[50]
[Eu
2
(L10)
3
]
20
181
26
[50]
[Eu
2
(L11)
3
]
20
177
34
[50]
[LaEu(L17)
3
]
6
þc
7
131
16
[64]
[Eu
2
(L17)
3
]
6
þd
10
125
33
[64]
a
Full width at half height.
b
At 295 K.
c
Solid state sample.
d
Single crystal.
5
D
0
!
7
F
0
transition is forbidden, so that it is observed with a very faint intensity. It
features only one relatively symmetrical component, indicative of the two metal ion sites
having the same chemical environment. The
7
F
1
level is split into two components, A
2
and E (degenerate twice) and the magnitude of this splitting is directly proportional to the
strength of the crystal field (in fact it is proportional to the
B
0
ligand-field parameter
[62]). The additional splitting seen in the
E
component is indicative of a slight distortion
from the idealized
D
3
symmetry. The corresponding parameters are reported in Table 6.5.
The A
2
-E separation varies negligibly along the series with ligands
i
5-9, with an aver-
age value of 160(2) cm
1
. It increases significantly (10-13%) in [Eu
2
(L10)
3
], [Eu
2
(L11)
3
]
in which the benzimidazole moieties bear aromatic substituents. As far as distortion from
the idealized symmetry is concerned, the splittings lie in a narrow 26-31 cm
1
range for
all helicates with the exception of [Eu
2
(L6)
3
] (38 cm
1
) and [Eu
2
(L11)
3
] (34 cm
1
), two
compounds the ligands of which bear relatively large substituents on the benzimidazole
units. Altogether, these splittings point to relatively small deviations from the ternary
symmetry, which may well be induced by the freezing of the solutions.
Heterometallic 4f-4f
0
helicates have been obtained with the unsymmetrical ditopic lig-
ands sketched on Scheme 6.3. In solution, mixtures of several species are obtained but
heterometallic [LnLn
0
(L)
3
]
6þ
are always favoured [7,65,66]. A luminescence study is
only available for helicates with ligand L17, with which crystal structures for the pairs of
lanthanoids LaEu, LaTb, PrEr, PrLu and EuEu could be solved. These structural data
show that: (i) the ligands wrap around the metal centres in a head to head to head (
HHH
)
arrangement, (ii) a pseudo axial
C
3
symmetry is maintained in the edifices, (iii) the Ln-X
distances for a given Ln
III
ion in a given coordination site are the same as in reference
homometallic edifices and (iv) the smaller ion is coordinated in the N
6
O
3
cavity [64].
The energy of the
5
D
0
!
¼
7
F
0
transition for [LaEu(L17)
3
]
6þ
is within 1 cm
1
of the one
reported for [Eu
2
(L4)
3
]
6þ
(Table 6.1): 17 230 cm
1
and in agreement with the calculated
value of 17 235 cm
1
. However, both D
E
(A
2
-E) and D
E
(E-E) are substantially smaller.
In the case of a single crystal of [Eu
2
(L17)
3
]
6þ
only one
5
D
0
!
7
F
0
transition is observed
at 17 232 cm
1
(compare 17 236 and 17 232 cm
1
for helicates with L3 and L4,
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