Slow paramagnetic relaxation of some pseudo-tetrahedral ferrous compounds in zero field

Slow (zero field) paramagnetic relaxation is reported for undiluted tetrahedral ferrous monomers for the first time. The Moessbauer spectra of Fe(2,9 di-CH3-phenanthroline)(NCS)2 and Fe(2,9 di-CH3 4,7 di-phenyl-phenanthroline)(NCS)2 exhibit magnetic hyperfine splitting in the range 20 K to 1.6 K while the corresponding chlorides are rapidly relaxing paramagnets at all temperatures in zero field. Low temperature susceptibility and Moessbauer results indicate that D changes sign from positive to negative from the Cl- to NCS- respectively with the foregoing ligands. Moessbauer spectra to 1.6 K confirm rapid paramagnetic relaxation for other pseudo-tetrahedral analogues (e.g. Fe(biquinoline)X2, X=Cl-, NCS-). The slow relaxation is then peculiar to the 2,9 di-CH3-phenanthroline system