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Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited : a new assignment of structural changes and crystal-field effects 25 years later

Authors
Ouali, Nadjet
Published in Inorganic Chemistry. 2002, vol. 41, no. 6, p. 1436-1445
Abstract Variable-temperature 1H and 13C NMR measurements of the D3-symmetrical triple-helical complexes [Ln(L1-2H)3]3- (L1 = pyridine-2,6-dicarboxylic acid; Ln = La−Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)3]3- (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La−Eu) and one for small lanthanides (Ln = Tb−Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter r 2 parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb−Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La−Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.
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Research group Groupe Weber
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OUALI, Nadjet et al. Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited : a new assignment of structural changes and crystal-field effects 25 years later. In: Inorganic Chemistry, 2002, vol. 41, n° 6, p. 1436-1445. https://archive-ouverte.unige.ch/unige:3223

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Deposited on : 2009-09-21

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