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) molecules coordinate with iron(II) as tridentate ligands forming complexes of distorted octahedral symmetry. The imino-hydrogen of the coordinated 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 the different species were performed and the derived parameters found to be in good agreement with the measured ones. The solid deprotonated complexes, showing in general thermal induced spincrossover, are predominantly at their low-spin (LS) state at room temperature in solid state. In contrast, a pronounced spin-crossover can be observed in methanolic solution. Thermodynamic parameters for [Fe(H-bzimpy)(2)](2+) and [Fe(H-bzimpyH(-1))(2)] are derived in methanolic solution as Delta H and 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/mol, respectively.