Electron paramagnetic resonance spectra and spin relaxation of manganese(III) complexes with Acetylacetonates and pyrophosphate ligands.

Zero-field splittings were measured by high-field and high-frequency EPR for the non-Kramers S = 2 Mn(III) complexes of acetylacetonates, yielding D values of -4.35 cm-1 for Mn(acac)3 (acac = 2,4-pentanedionato) and - 4.47 cm-1 for Mn(mesacac)3 (mesacac = 1,3-bis(2,4,6-trimethylphenyl)-propane-1,3-dionato)). Simulations of the X-band spectra of the Mn(III) pyrophosphate complex yielded D = 1.1 cm-1. At 11,500 G (g ∼ 0.60) the optimum pulse power for the pyrophosphate complex was about 17 dB (50 times) higher than predicted for S = 2 with g = 2, suggesting the importance of the very small effective g value for this complex. Relaxation times (T1 and Tm) of these three complexes were measured by pulsed X-band EPR in the normal perpendicular mode. These are the first direct relaxation measurements of discrete coordination complexes of Mn(III). Based on comparison with M(IV) complexes and aquo Mn(II), increase in 1/T1 correlates with increase in zero field splitting D, consistent with major contributions from an Orbach process with energy of the excited state proportional to the ZFS D value.