¹H NMR in Solution and Solid State Structural Study of Lanthanide(III) Cryptates

Abstract

We present here a detailed structural comparison, both in the solid state and in aqueous solution, of a complete series of lanthanide cryptate complexes of a Schiff base axial macrobicyclic ligand L of general formula [LnL]-[NO 3 ] 3 ‚xH 2 O (Ln ) La-Lu, Y); the macrobicyclic receptor L is an azacryptand N[(CH 2 ) 2 NdCH-R-CHdN-). The crystal structures of the Ce, Nd, and Eu complexes, chemical formulae [CeL(NO 3 )](NO 3 ) 2 ‚1.5H 2 O‚0.5CH 3 CH 2 OH (3), [NdL(NO 3 )](NO 3 ) 2 ‚3H 2 O (5), and [EuL(NO 3 )](NO 3 ) 2 ‚H 2 O‚ CH 3 OH (7), as well as that of [YL(NO 3 )][Y(NO 3 ) 3 (H 2 O) 2 EtOH](NO 3 ) 2 .EtOH‚CH 3 CN (16), have been determined by single-crystal X-ray crystallography. The four crystals crystallize in the triclinic space group (14)°for 16. In light of their crystal structures, it can be stated that all of them adopt very similar structures, with the nine-coordinated metal ion bound asymmetrically to seven donor atoms in the ligand cavity and also to two oxygen atoms of a bidentate nitrate anion. The macrobicycle cavity adapts to the lanthanide contraction, while preserving the pseudo-triple-helix conformation around the metal ion. The coordination geometry of the metal atom is best considered as a slightly distorted monocapped dodecahedron. The aqueous solution structures of the paramagnetic complexes were thoroughly characterized from the proton NMR LIS and LIR data, with particular attention to the changes induced by the lanthanide contraction, and agree quite well with the crystal structures of the Nd and Y complexes. The experimental Ln-donor distances decrease progressively along the lanthanide series both in the solid and solution structures, but no drastic structural changes occur. The gradual contraction and distortion of the coordination polyhedron along the series cause a variation of the crystal field parameter A 2°< r 2 > and the hyperfine constants A i of the lanthanides in the middle of the series, leading to "breaks" in the contact-pseudo-contact shift separation plots of the proton LIS values. However, this affects only slightly the geometric terms G i of the protons and not at all their R ik ratios. The conformational rigidity of the five-membered chelate rings formed by the metal-bound ethylenediamino moieties of the bound cryptand increases upon lanthanide contraction. The ∆G q value for the δ T λ conformational interconversion process of those rings is 70 ( 3 kJ for the Y complex. (1) Alexander, V. Chem. ReV. 1995, 95, 273. (2) Guerriero, P.; Tamburini S.; Vigato P. A. Coord. Chem. ReV. 1995, 139, 17. (3) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993, 123, 201. (4) The term axial macrobicycle describes symmetrical molecules with coaxial arrangement of two tripodal subunits linked by three identical bridges. See: Lehn, J.-M.

Mono- and bimetallic lanthanide(III) phenolic cryptates obtained by template reaction: solid state structure, photophysical properties and relaxivity †

“…27 In our case, the large nephelauxetic effect produced by the imine N atoms of ligand L is in agreement with the shorter mean Eu-N(imine) bond distance found for the [EuL(NO 3 )] 2ϩ cation, 2.57 Å, compared with the Eu-N(amine) of 2.71 Å. 17 Moreover, large contact contributions have been evidenced for imine protons in the 1 H-NMR spectrum of the Eu cryptate. 17 Under excitation through the ligand levels, 10 Ϫ3 M solutions of 2 in H 2 O and D 2 O (pH/D = 6.5) display an emission spectrum composed of a broad band arising from the 1 ππ* state as well as weak bands from the Eu() ion.…”

“…17 Moreover, large contact contributions have been evidenced for imine protons in the 1 H-NMR spectrum of the Eu cryptate. 17 Under excitation through the ligand levels, 10 Ϫ3 M solutions of 2 in H 2 O and D 2 O (pH/D = 6.5) display an emission spectrum composed of a broad band arising from the 1 ππ* state as well as weak bands from the Eu() ion. Using Horrock's equation for the calculation of the number of co-ordinated water molecules, 23 …”

The First Self‐Assembled Trimetallic Lanthanide Helicates Driven by Positive Cooperativity

“…For Ln Dy ± Yb, the experimental 1 H NMR spectra do not fit those predicted with Equation (11), but we cannot ascribe this deviation to an abrupt structural change, since variations of the crystal-field parameters near the middle of the lanthanide series, sometimes referred to as the gadolinium break effect, [41] are likely to occur in [Ln 3 (L7) 3 ] 9 , as previously observed for related homo-and heteropolymetallic triplestranded helicates. [17,41] More sophisticated mathematical analyses using novel crystal-field independent techniques [36,42] extended to trimetallic complexes with different crystal-field parameters will be discussed elsewhere, [43] but they eventually confirm isostructurality along the complete lanthanide series for [Ln 3 (L7) 3 ] 9 (Ln Ce-Yb), together with an abrupt variation of the crystal-field parameters between Tb and Dy.…”

Analysis of lanthanide induced NMR shifts using a crystal field independent method: application to complexes of tetraazamacrocyclic ligands

“…(5) does not depend on crystal field parameters, a plot of D a / S z versus D b / S z is linear with slope R ab and intercept (F a (/R ab F b ) if there is no change in the hyperfine coupling constants and the complexes are isostructural [22]. Any deviation from linearity for such plots along the lanthanide series can be safely ascribed to structural changes affecting R ab (and thus G a and G b ).…”