Correlation of optical and NMR spectral information with coordination variation for axially symmetric macrocyclic Eu(III) and Yb(III) complexes: Axial donor polarisability determines Iigand field and cation donor preference

In Bleaney's theory of magnetic anisotropy, the second-order crystal field coefficient, B _o ², is predicted to determine the dipolar NMR shift of paramagnetic lanthanide complexes in solution. This parameter has been measured directly, by analysing the europium emission spectra for a series of eight- and nine-coordinate axially symmetric complexes based on cyclen including aza-carboxylate ligands (e.g. DOTA), phosphonates (DOTP), phosphinates and several carboxamides (e.g. DOTAM). For both Yb and Eu complexes with a common coordination number and geometry (square antiprism (SAP) or twisted square antiprism (TSAP)), the dipolar NMR shift correlates very well with this parameter, which also determines the sign and magnitude of a major CD band in the near-IR CD spectra of a series of enantiopure Yb complexes. Measurements of the free energy change associated with axial ligand exchange in a cationic europium tetraamide complex, [Eu(DOTAMPh)](CF ₃SO ₃) ₃ supported by a simple electrostatic perturbation model, have been interpreted in terms of a predominant donor atom polarisation model which affords a simple assessment of Ln ion donor atom preference and ranks the axial second-order ligand field coefficient.