Spin-crossover complexes in solution,I. Substitutional lability of [Fe(bzimpy)2](ClO4)2

Summary The iron(II) complexes of the nitrogen donor ligand 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy) have been investigated in nonaqueous solvents using various spectrophotometric methods. Measurements in methanol at 20°C revealed thatbzimpy can act as a bi-or tridendate ligand towards iron(II) in this solvent, forming [Fe(bzimpy)]²⁺ (tridendate, logK=5.54), [Fe(bzimpy)₂]²⁺ (bistridentate, logK=4.12) and [Fe(bzimpy)₃]²⁺ (tris-bidentate, logK=3.85). The pronounced thermochromism of the compound is shown to be the result of both spin-crossover and dissociation equilibria. Furthermore, in solvents of higher donor numbers (30), partial or complete deprotonation of the complex [Fe(bzimpy)₃]²⁺ is observed. Triethylamine added stepwise to methanolic solutions causes successive deprotonation.This paper is dedicated to Professor Dr. Wolfgang Beck on the occasion of his 60th birthday with warmest personal wishes