Complexes between the tetrapyridyl pendant-armed macrocyclic ligand (L) and the trivalent lanthanide ions have been synthesized, and structural studies have been made both in the solid state and in aqueous solution. The crystal structures of the La, Ce, Pr, Gd, Tb, Er, and Tm complexes have been determined by single-crystal X-ray crystallography. In the solid state, all the cation complexes show a 10-coordinated geometry close to a distorted bicapped antiprism, with the pyridine pendants situated alternatively above and below the main plane of the macrocycle. The conformations of the two five-membered chelate rings present in the complexes change along the lanthanide series. The La(III) and Ce(III) complexes show a lambdadelta (or deltalambda) conformation, while the complexes of the heavier lanthanide ions present lambdalambda (or deltadelta) conformation. The cationic [Ln(L)]3+ complexes (Ln = La, Pr, Eu, Tb, and Tm) were also characterized by theoretical calculations at the density-functional theory (DFT) B3LYP level. The theoretical calculations predict a stabilization of the lambdalambda (or deltadelta) conformation on decreasing the ionic radius of the Ln(III) ion, in agreement with the experimental evidence. The solution structures show a good agreement with the calculated ones, as demonstrated by paramagnetic NMR measurements (lanthanide induced shifts and relaxation rate enhancements). The 1H NMR spectra indicate an effective D2 symmetry of the complexes in D2O solution. The 1H lanthanide induced shifts (LIS) observed for the Ce(III), Tm(III), and Yb(III) complexes can be fit to a theoretical model assuming that dipolar contributions are dominant for all protons. The resulting calculated values are consistent with highly rhombic magnetic susceptibility tensors with the magnetic axes being coincident with the symmetry axes of the molecule. In contrast with the solid-state structure, the analysis of the LIS data indicates that the Ce(III) complexes present a lambdalambda (or deltadelta) conformation in solution.

We report a complete set of magnetic susceptibilities of lanthanide complexes with a macrocyclic ligand based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing four hydroxyethyl pendant arms (L(1)). The [LnL(1)](3+) complexes are isostructural along the lanthanide series from Ce(3+) to Yb(3+), with the only structural change observed along the series being the monotonous shortening of the Ln-donor distances due to lanthanide contraction. The (1)H NMR spectra point to a D2 symmetry of the [LnL(1)](3+) complexes in aqueous solution, which provides a unique opportunity for analysis of the rhombic magnetic anisotropies with an unequivocal location of the magnetic axes. The contact contributions for the observed paramagnetic shifts have been estimated with density functional theory calculations on the [GdL(1)](3+) complex. Subsequently, the pseudocontact shifts could be factored out, thereby giving access to the axial and rhombic contributions of the magnetic susceptibility tensor. Our results show that the calculated magnetic anisotropies do not follow the trends predicted by Bleaney's theory, particularly in the case of Ho(3+) and Er(3+) complexes.

Recently developed dynamic nuclear polarization (DNP) technology offers the potential of increasing the NMR sensitivity of even rare nuclei for biological imaging applications. Hyperpolarized 89 Y is an ideal candidate because of its narrow NMR linewidth, favorable spin quantum number (I=½) and long longitudinal relaxation times (T 1 Graphical abstractCorrespondence to: Carlos Platas-Iglesias; A. Dean Sherry. Supporting information and ORCID(s) for the author(s) for this article are available on the WWWundergttp://dx.doi.org/. HHS Public Access