Lanthanide(III) chelates of DOTA exist in the two interconverting diastereomeric forms of capped square antiprism and twisted capped square antiprism. Lanthanide-DOTP complexes have very rigid chelate structure with the tetraazacyclododecane ring locked in one conformation.
The fourteen elements that follow lanthanum in the periodic table are known as lanthanides. They include the following elements: cerium, Ce, praseodymium, Pr, neodymium, Nd, promethium, Pm, samarium, Sm, europium, Eu, gadolinium, Gd, terbium, Tb, dysprosium, Dy, holmium, Ho, erbium, Er, thulium, Tm, ytterbium, Yb, and lutetium, Lu. The lanthanides are chemically very similar to each other and most often they form ionic compounds in which they occur as trivalent cations. An important feature of the lanthanide(III) ions is the lanthanide contraction, which is the decrease in the ionic radius of lanthanide(III) ions with increasing atomic number. A large number of lanthanide complexes are known. Lanthanides form highly ionic complexes with monodentate ligands such as halogenides and water. Oxoanions such as nitrate, sulfate and carbonate can act as bidentate ligands and form complexes with lanthanides with high coordination numbers. The octadentate ligand DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) forms isostructural complexes with the lantahide(III) and yttrium(III). These complexes crystallize with a capped square antiprism geometry where the basal plane is occupied by four amine nitrogens of the macrocycle, the capped plane was occupied by four carboxylate oxygens of the acetate sidearms, and the capping position is occupied by a water molecule. The twist angle between the basal and capped planes is approximately 39°. The lanthanum DOTA complex however crystallizes with a twisted capped square antiprism geometry where the twist angle between the basal and capped planes is approximately 22°. There are several structural features common to the lanthanide(III) complexes of ligands derived from cyclen (1,4,7,10-tetraazacyclododecane). In these structures the macrocycle adopts a square [3333] where the basal plane is occupied by four amine nitrogens of the macrocycle in square antiprismatic arrangement of donor atoms. The opposite face of the prism is usally occupied by a planar arrangement of oxygen donors (carboxylate, water, hydroxyalkyl, amide, or phosphinate). In aqueous solution, lanthanide(III) chelates of DOTA exist in the two interconverting diastereomeric forms of capped square antiprism and twisted capped square antiprism . The ratio of the two forms depends on the size of the lanthanide cation. Pairs of enantiomers exist for the major isomer and for the minor isomer that can interconvert by arm rotation or ring inversion. Lanthanide-DOTP complexes have very rigid chelate structure with the tetraazacyclododecane ring locked in one conformation. The barrier to inversion appears to be much higher than for DOTA and no fluxional behavior was observed for the methylenephosphonate sidearms. However, phosphonate esters and phosphinates have clockwise and counterclockwise wrapping isomers.