Aspects of NMR Characterization of Metallacrowns

Praseodymium Metallacrown-Based NMR Probe for Enantioselective Discrimination of Mandelate Anions in Water

“…Among the numerous metallacrown-based compounds, the family of 15-MC-5 includes intriguing examples of water-soluble polynuclear metallamacrocyclic lanthanide complexes. − In these water-soluble Ln­(III)–Cu­(II) aminohydroximate metallacrowns (MC), the nearly planar neutral metallamacrocycle bearing five [Cu­(II)–N–O] repeat units surrounds the central Ln 3+ ion, whereas the open axial site of the lanthanide coordination sphere is filled with weakly bound inner-shell water molecules. − Due to this unique structural feature the family of tetraaqua Gd­(III) 15-MC-5 metallacrowns has been suggested as a model for design of efficient high-field MRI contrast agents. , It should be noted that the paramagnetic lanthanide systems have traditionally been investigated by NMR spectroscopy, starting with the classic “lanthanide shift reagents.” − However, there are very few papers devoted to 15-MC-5 complexes, which can act as shift agents. ,, The use of the Ln­(III)–Cu­(II) 15-MC-5 complexes as lanthanide shift reagents is problematic because complicated exchange interactions between five paramagnetic Cu­(II) ions and Ln­(III) causes severe line broadening. Nevertheless, recently we have demonstrated that the Pr­(III) and Nd­(III) metallacrowns can exhibit clearly defined comparatively narrow NMR signals. , This can be applied to chiral recognition of enantiomers using chiral 15-MC-5 synthesized according to a simple protocol. , Since elucidation of the metallacrown structure more than 30 years ago by Pecoraro and co-workers, it has been established that chiral metallacrowns can be easily prepared by the reaction of resolved amino hydroxamic acids (e.g., l -alanine-, l -phenylalanine-, and l -tyrosine hydroxamic acid). , The beauty of this strategy lies in the judicious choice of a side chain functional group that can lead to the formation of a chiral metal crown.…”

Insights on the Structure in Solution of Paramagnetic LnIII/GaIII 12-Metallacrown-4 Complexes Using 1D 1H NMR and Model Structures

“…As recently reviewed in literature, NMR can be a powerful tool for the evaluation of structural features also for highly paramagnetic MC complexes . Also, we recently and successfully applied the “all lanthanides” method for the analysis of the NMR features of a series of Mn III paramagnetic complexes Ln III Na I (OAc) 4 [12-MC MnIII(N)Shi -4]­(H 2 O) 4 ·6DMF (AcO – = acetate ion), which allowed us to clarify several structural features of the complexes in solution .…”

“…32 As recently reviewed in literature, NMR can be a powerful tool for the evaluation of structural features also for highly paramagnetic MC complexes. 33 Also, we recently and successfully applied the "all lanthanides" method for the analysis of the NMR features of a series of Mn III paramagnetic complexes Ln III Na I (OAc) 4 [12-MC MnIII(N)Shi -4](H 2 O) 4 •6DMF (AcO − = acetate ion), which allowed us to clarify several structural features of the complexes in solution. 31 While this was a significant achievement, the presence of highly paramagnetic Mn III complexes influenced the magnetic properties of the Ln III and therefore a diamagnetic ring metal analogue is necessary to further develop the theory of this system.…”