EXPERIMENTAL SPIN-CROSSOVER INVESTIGATIONS ON CHARGED AND NEUTRAL IRON(II) COMPLEXES WITH 4-SUBSTITUTED-2,6-BIS-(BENZIMIDAZOL-2'-YL) PYRIDINE

4-X-2,6-bis-(benzimidazol-2'-yl)-pyridine (X-bzimpy; X = H, Cl, OH) mo lecules coordinate with iron(II) as tridentate ligands forming complex es of distorted octahedral symmetry. The imino-hydrogen of the coordin ated ligand of the complexes, [Fe(X-bzimpy)(2)](2+) is deprotonated in triethylamine solution. The deprotonated complexes, [Fe(X-bzimpyH(-1) )(2)] as well as [Fe(X-bzimpy)(2)](2+) were investigated by UV-VIS and FT-IR, Moessbauer spectroscopy. Quantum-mechanical calculations of th e different species were performed and the derived parameters found to be in good agreement with the measured ones. The solid deprotonated c omplexes, showing in general thermal induced spincrossover, are predom inantly at their low-spin (LS) state at room temperature in solid stat e. In contrast, a pronounced spin-crossover can be observed in methano lic solution. Thermodynamic parameters for [Fe(H-bzimpy)(2)](2+) and [ Fe(H-bzimpyH(-1))(2)] are derived in methanolic solution as Delta H an d Delta S as 22.7 +/- 0.6, 25.6 +/- 1.2 kJ/mol and 85.1 +/- 2.1, 92.8 +/- 4.3 J/K/mol, respectively. In solid state for [Fe(H-bzimpy)(2)](2), Delta H and Delta S were 13.3 +/- 2.4 kJ/mol and 42.5 +/- 8.6 J/K/m ol, respectively.