Homoleptic versus Heteroleptic Formation of Mononuclear Fe(II) Complexes with Tris-Imine Ligands

We show a marked tendency of Fe­(II) to form heteroleptic [Fe­(L)­(L′)]­(ClO₄)₂ complexes from pairs of chelating tris-imine 3bpp, tpy, or 2bbp ligands. New synthetic avenues for spin crossover research become thus available, here illustrated with three new heteroleptic compounds with differing magnetic behaviors: [Fe­(H₄L1)­(Cl-tpy)]­(ClO₄)₂·C₃H₆O (1), [Fe­(H₂L3)­(Me3bpp)]­(ClO₄)₂·C₃H₆O (2), [Fe­(H₄L1)­(2bbp)]­(ClO₄)₂·3C₃H₆O (3). Structural studies demonstrate that 1 is in the low-spin (LS) state up to 350 K, while complexes 2 and 3 are, by contrast, in the high-spin (HS) state down to 2 K, as corroborated through magnetic susceptibility measurements. Upon exposure to the atmosphere, the latter exhibits the release of three molecules of acetone per complex, turning into the solvent-free analogue [Fe­(H₄L1)­(2bbp)]­(ClO₄)₂ (3a), through a single-crystal-to-single-crystal transformation. This guest extrusion process is accompanied by a spin switch, from HS to LS.