Electron Paramagnetic Resonance Spectra of Pentagonal Bipyramidal Gadolinium Complexes.

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 5 h 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.