isomers of the DOTA complexes, the distances between the O4and N4 planes are longer and the structures are more flexible. Thepresence of even one phosphorus acid pendant arm(in DO3AP orDO3APR, Chart 2) induces the formation of complexes with theTSA/TSA arrangement in the solid state.64–66The opening angle w (the O–Ln–O angle between two transannularoxygen atoms, Fig. 3) was found (based on the crystalstructures) to be a crucial parameter for coordination of thewater molecule.20 When the angle is higher than 135◦, water iscoordinated, but if it is lower, then the water is not bound.In addition to the pendant forming five-membered chelates, afew ligands with pendants forming six-membered chelates weresynthesized and their complexes were studied. The structure ofthe [Eu(do3amBnO–N)(H2O)]2+ complex (DO3AMBnO–N = 10-(2-hydroxy-5- nitrobenzyl)-1,4,7,10- tetraazacyclododecane-4,7,10-triacetamide; Chart 2) shows the SA isomer in the solid state.69In addition, the distance between the O4 and N4 planes is longer(2.486A˚ ) than that usual for the SA complexes (<2.4A˚ ), and thevalue of the opening angle 136◦ is just on the border for watercoordination. Similar SA/SA species were found in the solidstate for complexes of 10-[(1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3ApyNO; Chart 2).SA isomers withO4–N4 distances of 2.503, 2.482 and 2.451A˚ werefound in structures of the [Dy(do3apyNO)],70 [Tm(do3apyNO)]71and [Yb(do3apyNO)]71 complexes, respectively. All complexes haveopening angles w of about 125◦. Water is coordinated in the[Nd(do3apyNO)(H2O)] complex with an opening angle of 140◦and an O4–N4 distance of 2.418A˚ .71 Based on these data, it seemsthat the presence of at least one six-membered chelate ring in thecomplexes of the DOTA-like ligands favours the formation of theSA isomers.72The La(III) complex of the ligand having all four acetate pendantarms of DOTA replaced by 2-carboxyethyl groups is the SAisomer with the N4–O4 distance 2.853 A˚ . So, the La(III) ion isdeeply embedded in the cavity and the opening angle is only 118◦.In addition, the lanthanide(III) complexes of this ligand show avery low stability in solution.73Structures with DTPA-like ligands In spite of a number of DTPA modifications, the numberof X-ray structures found in CSD is rather low (∼50).74 InNa2[Gd(bopta)(H2O)] (MultiHanceR , Chart 1), the DTPA-basedanion is coordinated to the Gd(III) ion with five oxygen atomsof five carboxylates and three nitrogen atoms forming a distortedtricapped trigonal prism TTP (Fig. 4).42 The coordinationpolyhedron is completed with a water molecule. InMn[Gd(dtpa)(H2O)]·2H2O, the complex anion also shows thesame molecular structure75 as well as in the analogous Ag(I)76 andaminoguanidinium77 salts. The coordination sphere of gadolinium(III) in contrast agent MS-325 (VasovistR , Chart 1) reportedrecently shows the same features.78 Analogous structures werefound in DTPA complexes with other lanthanide(III) ions. Insome other structures, e.g. in (NH4)4[Gd2(dtpa)2]·6H2O79 andin guanidinium salt,77 the complex is dinuclear due to twocarboxylates bridging themetal centres of the monomeric units. Insuch dimeric complexes,water is not coordinated. In the structuresofDTPA-bis(amides), dimer formation is less pronounced, and thecoordination polyhedron is similar to complexes of DTPA itself.Fig. 4 Structure of Ln(III)-DTPA complexes. Oxygen atoms arered, nitrogen atoms blue, carbon atoms gray and lanthanides greenAll the structures are distorted; nevertheless, some commonstructural features are evident from their comparison. Oxygenatoms O2, O3,O4andO5 are in a plane (see Fig. 4).The lanthanideis placed close to the plane (∼0.7 A˚ ), the distance is close to thevalue found for bothTSAand SAisomers of theDOTAcomplexes.The opening angles O3–Ln–O5 and O2–Ln–O4 are different, 140◦and 150◦, respectively. The Ln–O and Ln–N distances fall in theexpected range 2.5–2.6 A˚ except for the Ln–N1 distance, whichis systematically longer (2.75 A˚ ) in all complexes. In solution,a fast exchange associated with alternation of acetates as wellas a change in ethylene bridge conformation occur, which leadsto an equilibrium between two enantiomers. In all complexes ofthe DTPA-like ligands, only coordination number 9 was found,probably due to systematically higher values of the opening anglesand a larger space above the O4 plane.The skeleton of the parent DTPA was modified in order to tunerelaxometric properties of the Gd(III) complexes. Such modificationwas, e.g. replacement of carboxylic groups by phosphinic orphosphonic group (DTTAP or DTTAPR, Chart 3),80 of acetate bycarboxyethylene (DTTA-N-CE or DTTA-N’-CE, Chart 3)82 orof ethylene by 1,3-propylene (EPTPA, Chart 3).81,82 However, thecrystal structures of complexes with such ligands have not beenpublished yet.Structures with DO3A-like ligandsDO3A (Chart 2) is a prototype of heptadentate ligand forminglanthanide(III) complexes with two coordinated water moleculesin aqueous solution. The coordination mode of DO3A tolanthanide(III) ions is analogous to that found for the DOTAcomplexes. In the solid state, the complexes are mostly SA isomerswith distances between the N4 and O3 planes of about 2.35 A˚and coordination bond lengths in the usual range. The complexof the Gd(III) ion with DO3A was isolated in the solid state astrimer in which three [Gd(do3a)] units are connected througha carbonate anion and no water molecule is coordinated.83 Thecomplex of DO3MA (Chart 2) forms a dimeric species in the solidstate, with combined SA and TSA configurations.84 The SA unitcontains two coordinated water molecules. In the TSA unit, two“free” coordination sites are occupied by bidentately coordinatedacetate arms from the SA unit and, thus, no directly coordinatedwater molecule is present. More structures were published for lanthanide(III) complexes of DO3A amide derivatives. The expected
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