Chemistry Reference
In-Depth Information
O
Me
2
N
O
O
O
NMe
2
N
N
N
N
O
O
Gd
Gd
O
O
N
N
N
N
O
Me
2
N
O
O
O
NMe
2
[Gd.Ph-DOTA]
-
[Gd.DOTTA]
3+
Ph
Me
Ph
HN
O
O
N
Ph
O
P
O
O
O
Ph
N
N
P
O
Me
N
N
O
Gd
H
O
Gd
Me
O
N
N
N
N
NN
Gd
P
Ph
O
H
Ph
NN
O
O
P
NH
O
O
O
O
Me
O
Ph
Ph
[Gd.DOTAMPh]
3+
Gd. DO3A
Gadobenz
fIgure 8.4
The structures of [gd.Ph-dOTA]
-
, [gd.dOTTA ]
3+
, [gd.dOTAMPh]
3+
, [gd. dO3A], and gadobenz.
Changes in structure can influence the balance between these two diastereoisomers. With virtually all octadentate cyclen-
derived ligands, the SAP isomers predominate. Accordingly, they have also been referred to as M isomers (where M stands for
major), while the TSAP isomers are often referred to by m (where m stands for minor) [28]. Occasionally, the situation is reversed,
which can make the M/m descriptors confusing. For instance, dOTTA (Figure 8.4) predominantly forms TSAP isomers with
lanthanide ions (and we will discuss the consequences of this in detail later) [29]. It has also proved possible to simplify the picture
with regard to isomerism to the point where a single isomer forms in solution. For instance, dOTAMPh (Figure 8.4) [30, 31] exists
solely as a single SAP isomer in solution - the chiral amide pendant arms effectively favour just one form of the complex.
NMR spectroscopy can be utilised as a tool for determining the structure of most lanthanide complexes in solution. In the
presence of paramagnetic lanthanides, the chemical shifts of nearby nuclei are shifted with respect to their usual positions; this
is termed the lanthanide induced shift (LIS, Δω) and has contact (Δω
contact
) and pseudo-contact (Δω
pseudocontact
) contributions [32]:
∆∆ ∆
ωω ω
=
+
pseudocontact
(8.1)
contact
This can also be expressed as:
∆ω=+
SF Cg
Z
(8.2)
d
where S
Z
and C
d
are lanthanide specific terms, and F and g are ligand-specific parameters for contact and pseudo-contact
terms respectively. Rearrangement of these equations gives:
=
∆ω
S
C
S
+
gF
(8.3)
d
Z
Z
or
=
∆ω
C
S
C
+
Fg
Z
(8.4)
d
d
