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A Starburst‐Shaped Heterometallic Compound Incorporating Six Densely Packed Gd 3+ Ions
Author(s) -
Livramento João Bruno,
Sour Angélique,
Borel Alain,
Merbach André E.,
Tóth Éva
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200500969
Subject(s) - protonation , chemistry , lanthanide , coordination sphere , ligand (biochemistry) , crystallography , aqueous solution , coordination number , chelation , ion , inorganic chemistry , crystal structure , organic chemistry , biochemistry , receptor
Abstract The heterotritopic ligand [bpy(DTTA) 2 ] 8− has two diethylenediamine‐tetraacetate units for selective lanthanide( III ) coordination and one bipyridine function for selective Fe II coordination. In aqueous solution and in the presence of these metals, the ligand is capable of self‐assembly to form a rigid supramolecular metallostar structure, {Fe[Gd 2 bpy(DTTA) 2 (H 2 O) 4 ] 3 } 4− . We report here the physicochemical characterization of the dinuclear complex [Gd 2 bpy(DTTA) 2 (H 2 O) 4 ] 2− and the metallostar {Fe[Gd 2 bpy(DTTA) 2 (H 2 O) 4 ] 3 } 4− with regard to potential MRI contrast agent applications. A combination of pH potentiometry and 1 H NMR spectroscopy has been used to determine protonation constants for the ligand [bpy(DTTA) 2 ] 8− and for the complexes [Fe{bpy(DTTA) 2 } 3 ] 22− and [Y 2 bpy(DTTA) 2 ] 2− . In addition, stability constants have been measured for the dinuclear chelates [M 2 bpy(DTTA) 2 ] n − formed with M = Gd 3+ and Zn 2+ (log K GdL = 18.2; log K ZnL = 18.0; log K ZnHL = 3.4). A multiple field, variable‐temperature 17 O NMR and proton relaxivity study on [Gd 2 bpy(DTTA) 2 (H 2 O) 4 ] 2− and {Fe[Gd 2 bpy(DTTA) 2 (H 2 O) 4 ] 3 } 4− yielded the parameters for water exchange and the rotational dynamics. The 17 O chemical shifts are indicative of bishydration of the lanthanide ion. The exchange rates of the two inner‐sphere water molecules are very similar in the dinuclear [Gd 2 bpy(DTTA) 2 (H 2 O) 4 ] 2− and in the metallostar ( k ex 298 = 8.1±0.3×10 6 and 7.4±0.2×10 6 s −1 , respectively), and are comparable to k ex 298 for similar Gd III poly(amino carboxylates). The rotational dynamics of the metallostar has been described by means of the Lipari–Szabo approach, which involves separating global and local motions. The difference between the local and global rotational correlation times, τ lO 298 = 190±15 ps and τ gO 298 = 930±50 ps, respectively, shows that the metallostar is not completely rigid. However, the relatively high value of S 2 = 0.60±0.04, describing the restriction of the local motions with regard to the global one, points to a limited flexibility compared with previously reported macromolecules such as dendrimers. As a result of the two inner‐sphere water molecules, with their near‐optimal exchange rate, and the limited flexibility, the metallostar has a remarkable molar proton relaxivity, particularly at high magnetic fields ( r 1 = 33.2 and 16.4 m M −1 s −1 at 60 and 200 MHz, respectively, at 25 °C). It packs six efficiently relaxing Gd III ions into a small molecular space, which leads, to the best of our knowledge, to the highest relaxivity per molecular mass ever reported for a Gd III complex. The [bpy(DTTA) 2 ] 8− ligand is also a prime candidate as a terminal ligand for constructing larger sized, Fe II (or Ru II )‐based metallostars or metallodendrimers loaded with Gd III on the surface.