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Structure and Dynamics of Lanthanide(III) Complexes with an N ‐Alkylated do3a Ligand (H 3 do3a = 1,4,7,10‐Tetraazacyclododecane‐1,4,7‐triacetic Acid): A Combined Experimental and DFT Study
Author(s) -
RegueiroFigueroa Martín,
EstebanGómez David,
de Blas Andrés,
RodríguezBlas Teresa,
PlatasIglesias Carlos
Publication year - 2010
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201000334
Subject(s) - chemistry , cyclen , lanthanide , coordination sphere , dota , molecule , ligand (biochemistry) , carboxylate , crystallography , density functional theory , stereochemistry , crystal structure , ion , computational chemistry , chelation , inorganic chemistry , organic chemistry , biochemistry , receptor
Lanthanide(III) complexes with 10‐benzyl‐1,4,7‐tris(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane (H 3 L ) have been investigated as model compounds of N ‐alkylated Ln III –do3a complexes (H 3 do3a = 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid) with potential application in molecular imaging. The X‐ray structures of the [{Ln( L )(H 2 O)} 4 ] · 18H 2 O complexes (Ln = Eu or Tb) show that the metal ion is directly bound to the seven donor atoms of the ligand, with the nine‐coordination sphere completed by an oxygen atom of an inner‐sphere water molecule and an oxygen atom of a carboxylate group from a neighboring [Ln( L )] unit. This results in the formation of tetrameric units through the sharing of carboxylic groups between adjacent [Ln( L )(H 2 O)] complexes. Luminescence lifetime measurements recorded in H 2 O and D 2 O provide a number of coordinated water molecules ( q ) of 1.2, which indicates that the major species that exists in solution contains one inner‐sphere water molecule. The conformational properties of the [Ln( L )(H 2 O)] complexes (Ln = Gd or Lu) have been investigated by using density functional theory (DFT) calculations (B3LYP model). Our results indicate that the Δ(λλλλ) conformation is more stable than the Δ(δδδδ) one for both complexes. The interconversion between these two isomers may proceed either through the inversion of the five‐membered rings formed upon coordination of the 1,4,7,10‐tetraazacyclododecane (cyclen) unit, or through the stepwise rotation of the three acetate pendant arms. According to our calculations, the activation free energy for the arm‐rotation process (5.6 kcal mol –1 ) is much lower than that of the ring‐inversion path (14.6 kcal mol –1 ), which in turn is very similar to those determined experimentally for Ln III (dota) complexes. Thus, a low energy barrier for the arm‐rotation process appears to be responsible for the relatively fast isomer interconversion observed for Ln III ( N ‐alkylated do3a) complexes.