Electron Paramagnetic Resonance Spectra of Pentagonal Bipyramidal Gadolinium Complexes

[Image: see text] Gadolinium is a special case in spectroscopy because of the near isotropic nature of the 4f(7) configuration of the +3 oxidation state. Gd(3+) complexes have been studied in several symmetries to understand the underlying mechanisms of the ground state splitting. The abundance of information in Gd(3+) spectra can be used as a probe for properties of the other rare earth ions in the same complexes. In this work, the zero-field splitting (ZFS) of a series of Gd(3+) pentagonal bipyramidal complexes of the form [GdX(1)X(2)(L(eq))(5)](n+) [n = 1, X = axial ligands: Cl(–), (–)O(t)Bu, (–)OArF(5) or n = 3, X = (t)BuPO(NH(i)Pr)(2), L(eq) = equatorial ligand: Py, THF or H(2)O] with near fivefold symmetry axes along X(1)-Gd-X(2) was investigated. The ZFS parameters were determined by fitting of room-temperature continuous wave electron paramagnetic resonance (EPR) spectra (at X-, K-, and Q-band) to a spin Hamiltonian incorporating extended Stevens operators compatible with C(5) symmetry. Examination of the acquired parameters led to the conclusion that the ZFS is dominated by the B(2)(0) term and that the magnitude of B(2)(0) is almost entirely dependent on, and inversely proportional to, the donor strength of the axial ligands. Surveying the continuous shape measure and the X(1)-Gd-X(2) angle of the complexes showed that there is some correlation between the proximity of each complex to D(5h) symmetry and the magnitude of the B(6)(5) parameter, but that the deformation of the X(1)-Gd-X(2) angle is more significant than other distortions. Finally, the magnitude of B(2)(0) was found to be inversely proportional to the thermal barrier for the reversal of the magnetic moment (U(eff)) of the corresponding isostructural Dy(3+) complexes.