Critical analysis of the limitations of Bleaney's theory of magnetic anisotropy in paramagnetic lanthanide coordination complexes

Abstract

The origins of the breakdown of Bleaney's theory of magnetic anisotropy are described, based on an analysis of eleven different complexes of the second half of the 4f elements that form isostructural series.

Simultaneous Triple Imaging with Two PARASHIFT Probes: Encoding Anatomical, pH and Temperature Information using Magnetic Resonance Shift Imaging

“…The unusually large shifts for the Tm and Tb complexes do not follow the expected trend predicted by Bleaney's theory of magnetic anisotropy. Such behaviour is in agreement with recent observations that have highlighted the need to appreciate that this theory is not generally applicable, and is quite imprecise when the relative magnitude of the components of the magnetic susceptibility tensor varies from one lanthanide to another, within a common isostructural series …”

“…The synthesis of the target complexes, [HLn⋅L 1 ] − was undertaken using a convergent route in which the key heterocyclic moiety was introduced late in the scheme by an alkylation reaction at the fourth ring nitrogen position (Schemes and Schemes ). Nitration of 3‐ tert‐ butyl‐6‐methyl pyridine N ‐oxide gave the 4‐nitro derivative, 1 , and stepwise reduction, diazotization and trapping of the aryldiazonium ion with iodide afforded the p ‐iodo derivative, 3 . Thermolysis in acetic anhydride led to functionalisation of the proximate benzylic site generating the acetate ester, 4 .…”

“…To improve sensitivity they can be labelled with homotopic t Bu groups that undergo fast relaxation in the range 80 to 150 s −1 , over the field range 1 to 7 Tesla. The t Bu resonance, when it is located about 6.6 Å from the metal ion in an 8‐coordinate complex, can be tuned to resonate ±80 ppm from the water signal, by varying the Ln 3+ ion . By selecting the lanthanide ion from the series of five “fast‐relaxing” and “large‐shifting” rare‐earth ions (Tb, Dy, Ho, Er and Tm) according to the magnetic field to be used, it is possible to select a pair of complexes with similarly large, but opposite paramagnetic shifts and similar R 1 and R 2 values.…”

Facile One‐Pot Route toward Water‐Soluble Lanthanide–Copper–Glycinehydrox­imate 15‐Metallacrown‐5 Complexes

“…The splittings are not observed because of to the fast relaxation and moderate linewidth (approximately Ͼ 100 Hz for the methyl protons signal). Note, these data are not in line with Bleaney's theory of magnetic anisotropy and can be defined in terms of the limitations for paramagnetic lanthanide coordination complexes of lower symmetry recently described by Parker et al [19] Table 2. [11] To exclude the ferromagnetic component, we have chosen 4(La) complex because La III ion is diamagnetic; therefore, the paramagnetic effect should result exclusively from the copper(II) ions.…”

“…[15] Note that acetate ions have not been previously observed as binding to the Ln 3+ [4(Gd)]. [17][18][19] With this model in mind, we set out to examine the Ln-induced shifts of the lactate NMR proton signals in such water-soluble lanthanide metallacrowns based on glycinehydroxamic acid. The lactate was chosen as a representative essential oxyanion that exists as a metabolite in all living organisms.…”