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

Abstract

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 3 SO 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.

Europium(III) Macrocyclic Complexes with Alcohol Pendant Groups as Chemical Exchange Saturation Transfer Agents

“…Although the precise location of the donor atoms in the coordination polyhedron cannot by determined from the emission spectra of europium(III) complexes, the 5 D 0 → 7 F 1 and 7 F 2 emission bands at 594 and 613 nm, respectively, may be sensitive to changes in coordination number. 18 The intensity of the ΔJ = 2 band is considered to be "hypersensitive" to the europium(III) coordination environment, whereas the ΔJ = 1 band consists of two transitions, the separation of which is determined by the axial crystal field coefficient. Although the relative intensities of these bands remained the same as water was added (not shown), the overall intensity of the spectra did diminish slightly due to nonradiative quenching of the europium-(III) excited state by water.…”

Structural and Chiroptical Properties of the Two Coordination Isomers of YbDOTA-Type Complexes

“…Curiously, the sign of this band, although negative in the spectrum of Yb1, is positive in the spectrum of YbDOTAMPh•H 2 O. The work of Dickins and coworkers 24 has shown that in the case of YbDOTAMPh both the magnitude and sign of this band are highly sensitive to the nature of the axial ligand of the complex. However, Yb1 and YbDOTAMPh•H 2 O both possess the same axial ligand, water.…”

“…Acquiring the CD spectrum of a chiral Yb 3+ complex in the near-infrared region intimately probes the coordination polyhedron of the ion and may allow important information to be gathered. Although this technique has been applied to the study of Yb 3+ complexes of other octadentate cyclen based ligands, 7,11,[24][25][26] the isolation of the two coordination isomers of the single ligand NO 2 BnDOTMA offers a unique opportunity to further probe the intimate nature of these two coordination geometries.…”

“…The tetraamide derivatives of DOTA that bear a bulky amide substituent are wellknown to preferentially adopt SAP coordination geometries. 24,26,27 The chiral tetraamide complex (RRRR)-YbDOTAMPh has been extensively studied by NIR-CD spectroscopy and only adopts a SAP coordination geometry in solution. 24 The NIR-CD spectrum of Yb1 is shown with that of (RRRR)-YbDOTAMPh•H 2 O (Figure 6).…”

“…24,26,27 The chiral tetraamide complex (RRRR)-YbDOTAMPh has been extensively studied by NIR-CD spectroscopy and only adopts a SAP coordination geometry in solution. 24 The NIR-CD spectrum of Yb1 is shown with that of (RRRR)-YbDOTAMPh•H 2 O (Figure 6). There are broad similarities between the two spectra; in each spectrum four main NIR Cotton effects can be observed.…”