~1H NMR Spectroscopic Study of Paramagnetic Lanthanide(III) Texaphyrins. Effect of Axial Ligation

摘要

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(NO_3)_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.