solution, the ring interconversion d k or acetate arm rotation D
Klead to formation of four stereoisomers (two diastereoisomers
Ddddd/Kkkkk and Dkkkk/Kdddd; Fig. 2). Both diastereoisomers
differ in structural parameters, e.g. in the angle x formed by
the mutual rotation of the O4 and N4 planes (Fig. 3). In the
Dkkkk/Kdddd isomers, a rotation of ∼40◦ leads to the squareantiprismatic
isomer SA (ideal angle 45◦). This diastereoisomer
is also traditionally termed “MAJOR” (M) due to its higher
abundance in solutions of the [Gd(dota)(H2O)]− complex; in the
pair of Kkkkk/Ddddd enantiomers, a rotation of ∼24◦ corresponds
to the twisted square-antiprismatic isomer TSA (ideal angle 22.5◦)
or “minor” (m) isomer.61 Lanthanide(III) complexes of DOTA
form mostly SA isomers. The TSA isomers were observed in
structures of large ions such as lanthanum(III)62 or cerium(III)63 or
for small ions e.g. thulium(III);63 but without a coordinated water
molecule in the case of Tm(III) (such arrangement with no bound
water molecule is often denoted TSA or m). The corresponding
SA isomer (SA without coordinated water) was found only for
the small scandium(III) ion.63
The comparison of all the X-ray crystal structures of the lanthanide(
III) complexes ofDOTA, its amides and several derivatives
bearing one methylphosphonic/phosphinic acid group64–66 found
in the literature indicated the same structural properties of the
coordination polyhedron. The TSA/SA ratio is influenced by the
size of the lanthanide(III) ion and by the shape of the ligand cavity.
The geometry and the size of the complex cavities formed by these
ligands are mostly governed by rotation of the pendants. It was
shown that different orientations of the pendant arms lead to
different distances between the N4 and O4 planes (Fig. 3). The
lighter lanthanide(III) ions such as lanthanum(III) and cerium(III)
ions require a larger cavity and, thus, the N4–O4 distance is 2.5A˚
or longer and the formation of the TSA isomer is preferred in
their complexes. As the radius of the lanthanide(III) ion decreases
in the lanthanide series, the ion moves towards the N4 plane and,
consequently, the coordinated oxygen atoms in the O4 plane also
move closer to the N4 base and the structure switches to the SA
arrangement with the N4–O4 distance of 2.3–2.4A˚ .
Replacement of all carboxylic acid groups with phosphonic
or phosphinic acid groups leads to the tetraphosphorus DOTA
analogues, abbreviated DOTP and DOTPR (Chart 2). In the solid
state, lanthanide(III) complexes of such ligands show mostly TSA
isomers through the lanthanide series. The coordinated water
molecule (in the TSA isomer) was found only in structures of
the La(III) and Ce(III) complexes.67,68 The preferential TSA/TSA
isomer formation is probably caused by bulky phosphorus acid
pendants and the tetrahedral geometry of phosphorus atoms.
This leads to a smaller opening angle w and expulsion of a water
molecule fromthe first coordination sphere. In contrast to theTSA