Formation constants and thermodynamic properties of the spin-crossover Fe(II)-2,6-bis-(benzimidazol-2′-yl)pyridine complex

The tridentate ligand, 2,6-bis-(benzimidazol-2′-yl)pyridine (bzimpy) coordinates to iron(II) and forms the mono- and bis-complexes, [FeLS3]2+ (S=solvent) and [FeL2]2+ in 50% (v/v) PC/MeOH (PC=propanedilo-1,2-carbonate). The complexes are characterized by observed metal-to-ligand charge transfer (MLCT) bands with absorption maximum at 500.0±5.0 nm (εmax=200–700 l mol−1 cm−1) for [FeLS3]2+ and at 555.0±1.0 nm (εmax=5100–7500 l mol−1 cm−1) for [FeL2]2+. The complexation equilibria are studied at an extended temperature range of 4, 13, 20 and 32±1 °C in 50% (v/v) PC/MeOH. The values of formation constants (log10 β2=10.90–11.47) are calculated and found to decrease with increasing temperature. The thermodynamic parameters (ΔH and ΔS) for complexation equilibria are 50.52 kJ mol−1 and 43.28 J mol−1 K−1, respectively. Study at low temperature (4 °C) reveals the likely formation of a tris-complex, [FeL3]2+ in addition to [FeLS3]2+ and [FeL2]2+ in solution.