Crystal Structures of Lanthanide(III) Complexes with Cyclen Derivative Bearing Three Acetate and One Methylphosphonate Pendants

A series of lanthanide(III) complexes formulated as M[Ln(Hdo3ap)]·xH2O (M = Li or H and Ln = Tb, Dy, Er, Lu,
and Y) with the monophosphonate analogue of H4dota, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-methylphosphonic acid (H5do3ap), was prepared in the solid state and studied using X-ray crystallography. All of
the structures show that the (Hdo3ap)4- anion is octadentate coordinated to a lanthanide(III) ion similarly to the
other H4dota-like ligands, i.e., forming O4 and N4 planes that are parallel and have mutual angle smaller than 3°.
The lanthanide(III) ions lie between these planes, closer to the O4 base than to the N4 plane. All of the structures
present the lanthanide(III) complexes in their twisted-square-antiprismatic (TSA) configuration. Twist angles of the
pendants vary in the range between −24 and −30°, and for each complex, they lie in a very narrow region of 1°.
The coordinated phosphonate oxygen is located slightly above (0.02−0.19 Å) the O3 plane formed with the coordinated
acetates. A water molecule was found to be coordinated only in the terbium(III) and neodymium(III) complexes.
The bond distance Tb−Ow is unusually long (2.678 Å). The O−Ln−O angles decrease from 140° [Nd(III)] to 121°
[Lu(III)], thus confirming the increasing steric crowding around the water binding site. A comparison of a number
of structures of Ln(III) complexes with DOTA-like ligands shows that the TSA arrangement is flexible. On the other
hand, the SA arrangement is rigid, and the derived structural parameters are almost identical for different ligands
and lanthanide(III) ions.