1H NMR spectroscopic study of paramagnetic lanthanide(III) texaphyrins. Effect of axial ligation

Article abstract of DOI:10.1021/ja00113a016

Paramagnetic Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), and Yb(III) texaphyrins were studied in solution using ¹H NMR spectroscopic techniques. Key spectroscopic features for the dinitrate complexes LnTx(NO₃)₂ were assigned on the basis of 1D NOE, COSY, and ROESY experiments as well as line width and isotropic shift analysis. The observed isotropic shifts can be fit to theoretical models, assuming dipolar contributions are dominant for all but the imino protons. The resulting calculated values are consistent with highly rhombic magnetic susceptibility tensors for those paramagnetic lanthanide texaphyrins in which one of the molecular magnetic axes is roughly perpendicular to the macrocycle plane. For the dinitrate complexes, a change in the magnetic anisotropy was observed between the Ho(III) and Er(III) texaphyrin complexes, a phenomenon that is considered reflective of the changes in metal-centered axial ligation that occur as the lanthanide series is transversed. The derived contact shifts of the imino protons were found to follow well the expected theoretical dependence on 〈S_z〉. Exchange of both axial nitrate anions for phosphate-type ligands, confirmed by solution phase titration studies, brings about a drastic change in the observed spectral patterns. These changes can be accounted for fully by altering the magnetic susceptibility tensor so as to accommodate what are presumed to be changes in the effective crystal field parameters. Conformation of phosphate coordination in the solid state came from a single crystal X-ray diffraction analysis of the bis(diphenyl phosphate) adduct of Dy(III) texaphyrin. Crystals of (C₃₄H₃₈N₅O₂)Dy((C₆H ₅)₂-PO₄)₂?0.64(CH ₃OH) (DyTx(P1)₂), obtained by diffusion of diphenyl phosphate into methanolic DyTx(NO₃)₂ solution, were triclinic, space group P1, with a = 14.482(3), b = 14.664(3), and c = 15.580(3) ?, α = 69.544(15), β= 65.695(15), and γ = 70.377(15)°, V = 2751.1(11) ?³, Z = 2, Q_calc = 1.48 g/cm^-3. The structure refined to a final R(F²) = 0.109 for 644 parameters using 8390 reflections. The Dy(III) ion is seven-coordinate with five donor atoms being provided by the texaphyrin ligand and two by monodentate diphenyl phosphate ions. The Dy(III) ion is only 0.073 ? from the plane through the five nitrogen atoms of the macrocycle. The Dy(III)-O bond lengths are equivalent and average 2.228(5) ?. The Dy-N_pyrrole bond lengths average 2.363(3) ? while the Dy-N_imine bond lengths average 2.448(4) ?.