Structure and magnetism of electronically distorted iron(III) Schiff-base complexes

Abstract

The reaction of [Fe(salen)Cl][H₂salen =N,N′-bis(salicylidene)ethane-1,2-diamine] with an excess of L [imidazole (Him), 1-,2- or 4-methylimidazole (1-,2- or 4-mim)] in acetone resulted in the isolation of heterobisadducts, [Fe(salen)L(Cl)](L = Him, 1- or 2-mim) and a homobisadduct, [Fe(salen)L₂]Cl (L = 4-mim). The structures of [Fe(salen)(1-mim)Cl] and [Fe(salen)(4-mim)₂]Cl were determined by single-crystal X-ray diffraction: [Fe(salen)(1-mim)Cl], monoclinic, space group p2₁/c, Z= 4, a= 9.8510(10), b= 15.9830(10), c= 12.8350(10)Å, β= 107.610(10)°; [Fe(salen)(4-mim)₂]Cl, orthorhombic, space group Fdd2, Z= 8, a= 21.7650(10), b= 12.5040(10), c= 18.508(2)Å. The structures were refined by full-matrix least-squares methods on F² to R= 0.0537 and 0.0338 for 2609 and 1207 reflections, respectively. A significant difference between the two structures is that the dihedral angle between the two phenyl rings of the salen ligand is 17.5° for [Fe(salen)(1-mim)Cl] and 4.4° for [Fe(salen)(4-mim)₂]Cl. All of the complexes were characterized by variable-temperature magnetic susceptibility and Mössbauer spectroscopy. The heterobisadducts, which have a N₃O₂Cl donor set, exhibit an admixed electronic ground state whereas the homobisadduct is high spin. Another N₃O₂Cl donor set molecule has been found to exhibit a temperature-dependent spin equilibrium, ²T ⇌⁶A, as evidenced by variable-temperature magnetic susceptibility, ESR and Mössbauer spectroscopies.