SINGLET REVERSIBLE-ARROW QUINTET TRANSITION IN A 6-COORDINATE IRON(II) COMPLEX OF A TRIDENTATE BIS(PYRAZOLYL)PYRIDINE LIGAND

The bis(ligand)iron(II) complex [Fe(L1)2](ClO4)2.H2O(1) [L1 = 2,6-bis-(pyrazol-1-ylmethyl)pyridine] displays in the solid-state anomalous magnetic properties which are associated with a temperature-induced singlet (1A1) half arrow right over half arrow left quintet (5T2) transition. Mossbauer spectral data confirm this and reveal separate contributions from the singlet and quintet species with a strong temperature dependence of their relative intensities. Variable-temperature magnetic and Mossbauer studies establish that [Fe(L2)2](ClO4)2.2H2O (2) [L2 = 2-(3,5-dimethylpyrazol-1-ylmethyl)-6-(pyrazol-1-ylmethyl)pyridine] and [Fe(L3)2](ClO4)2.2H2O (3) [L3 = 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine] are purely high-spin species. For 1 three d-d transitions originating from the quintet and singlet state have been identified.