Electron paramagnetic resonance spectra and electron spin relaxation of six-coordinate lanthanide complexes with a...

X-band field-swept echo-detected spectra and electron spin relaxation times were measured for meridional pseudo-octahedral Ce³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Tb³⁺, Er³⁺, Tm³⁺, and Yb³⁺ complexes with a phosphine oxide ligand (tris-(3,5-dimethoxyphenyl) phosphine oxide) (OPR₃) and compared with 8- and 9-coordinate complexes. Data for Tb³⁺, Er³⁺, and Tm³⁺ were also acquired at Q-band. T₁ and T_m values for the OPR₃ complexes, measured by 3-pulse inversion recovery and 2-pulse Hahn echo, are short relative to commonly observed values for 3d transition metal complexes or the same ions in 8- or 9-coordinate complexes. The short echo dephasing times are attributed to the effect of vibronic modulation of the large anisotropy of zero field splittings, g values, and hyperfine interactions on the echo refocusing. Other than the S-state Gd³⁺, the longest T₁ values for all of the coordination environments are for Tm³⁺. Although T₁ for Er³⁺ is the shortest for the aquo, HOPO, and LanM complexes, it is not the shortest for the OPR₃ complex, for which it is unusually temperature dependent. The shortest T₁ values for the OPR₃ complexes are for Sm³⁺ and Tb³⁺, which is attributed to very low-lying excited states, that vary with coordination environment. The microwave power required to create a π/2 pulse is in most cases consistent with J being a good quantum number, but for Tb³⁺ J + I gives better agreement. In some spectra the pulse power is the same over the magnetic field range of a few hundred G to 13,000 G, but in other cases different pulse powers are needed for different magnetic fields, which is attributed to multiple electronic energy levels.