Among the Ln[15MC CuRha -5] metallacrowns (MC; Rha = R-substituted hydroximate ligands), the simple glycinehydroximate (Glyha) complex is presented as the primary model for studying various aspects of the guest interaction with the Ln III ion. We report a one-pot synthesis of several glycinehydroximate LnL[15MC CuGlyha -5] derivatives in the presence of calcium compounds. The complexes with biden-II Glyha -5](NO 3 ) 2 (3), and biden-[a] G.A. Razuvaev 5202 tate lactate Gd(CH 3 CHOCOO)[15MC Cu II Glyha -5]Cl 2 [4(Gd)]have been prepared and characterized by X-ray analysis. The lactate was chosen as an example of the essential oxyanions that exist as metabolites in all living organisms. We examined the lanthanide-induced NMR spectroscopy shifting ability of a series of lactate 4(Ln) metallacrown complexes (Ln = La, Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, or Dy). The corresponding Gd III complex was characterized by 1 H NMR spectroscopy relaxometric techniques in aqueous media. II Glyha -5](NO 3 ) 2 can be easily prepared by a onepot method in the presence of calcium compounds. [15] Herein, we have developed this synthetic strategy when pre-Scheme 2. Synthetic pathway to nitrate 1.
The development of contrast agents specifically designed for high‐field magnetic resonance imaging (MRI) is required because the relaxation efficiency of classic Gd(III) contrast agents significantly decreases with increasing magnetic field strengths. With an idea of exploring the unique structure of lanthanide (Ln) 15‐MC‐5 metallacrowns, we developed a series of water‐soluble Gd(III) aqua‐complexes, bearing aminohydroxamate (glycine, α‐alanine, α‐phenylalanine and α‐tyrosine) ligands, with increasing number of water molecules directly coordinated to the Gd(III) ion: Gd(H2O)4[15‐MCCu(II)Glyha‐5](Cl)3 (1(Gd)), Gd(H2O)4[15‐MCCu(II)Alaha‐5](Cl)3 (2(Gd)), Gd(H2O)3[15‐MCCu(II)Phalaha‐5](Cl)3 (3(Gd)) and Gd(H2O)3[15‐MCCu(II)Tyrha‐5](Cl)3 (4(Gd)). In these systems, the Ln(III) central ion is coordinated by five oxygen donor atoms of the ligands and three or four inner‐sphere water molecules. The X‐ray crystal structure of metallacrown Ln(H2O)3,4[15‐MCCu(II)Rha‐5]3+ agrees with density functional theory predictions. The calculations demonstrate that the exchange of coordinated water molecules can proceed easily, resulting in increased relaxivity parameters. The longitudinal relaxivities (r1) of 1(Gd)–4(Gd) in water at ultrahigh magnetic field of 9.4 T were determined to be 11.5, 14.8, 13.9 and 12.2 mM−1 s−1, respectively. The ability to increase the number of Ln(III) inner‐sphere water molecules up to four, the planar metallacrown structure and the rich hydration shell due to strong hydrogen bonds between the [15‐MC‐5] moiety and bulk water molecules provide new opportunities for potential MRI applications.
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