SPIN-TRANSITION BEHAVIOR OF TRANSITION-METAL COMPLEXES WITH 2,6-BIS-(BENZIMIDAZO1-2'-YL)PYRIDINE INDUCED BY DEPROTONATION OF THE COMPLEX

2,6-bis-(BenzimidazoI-2'-yl)-pyridine (bzimpy=H(2)L) acts as a bidentate ligand when combining with transition metal ions. The complexes [M(bzimpy)(2)](ClO4)(2) (M = Fe2+, Mn2+, Zn2+, Co2+, and Ni2+) were obtained as solids. The protonation constants (log K) for the ligand and the complexes were evaluated in 30:70 (v/v) H2O:EtOH at 293 K and at constant ionic strength of 0.12 M KCl. Coordination of the ligand to the metal ions leads to an increase of acidity of the imino-hydrogen of the benzimidazole group of the ligand as a function of the complex stability. Deprotonation leads to a spin-state transition (intermediate spin-state --> low-spin) of the iron(II)-complex, followed by a shift of the metal-to-ligand charge transfer band (MLCT) to lower energies (lambda(max) = 563 to 580 nm). The d-d absorption bands are found to shift to higher energies and the low-spin isomer is favoured at room temperature. An opposite shift of the MLCT band (lambda(max) = 563 to 557 nm) is observed when HClO4 is added to the complex solution, rendering the high-spin state of the complex more favourable.