Structural and ¹H NMR spectroscopic characterization of bis(N-isopropylsalicylaldiminato)iron(II)

Coordinatively unsaturated iron(II) species form reactive organometallic and coordination complexes and are integral to the reactivity of non-heme iron proteins and their synthetic analogues. Iron(II) Schiff base complexes have proven to be easily prepared and useful starting materials for such compounds. Here we report a detailed preparative procedure and the solid-state structure of the iron(II) complex of N-isopropylsalicylaldimine (L^iprH) based on the compounds first synthesized by Larkworthy (J. Chem. Soc., A (1968) 1048). The title compound is prepared by adding 2 equiv. of salicylaldehyde (sa1H) to Fe(O₂CCH₃)₂ in KOH-CH₃OH to produce a precursor formulated as Fe(sal)₂ which is reacted subsequently with isopropylamine in THF to form Fe(L^ipr)₂ in 34% isolated yield. A single crystal X-ray crystallographic study of Fe(L^ipr)₂ reveals a mononuclear complex with two bidentate salicylaldiminate ligands bound to an iron(II) atom in a tetrahedral coordination geometry. The ¹H NMR spectrum of Fe(L^ipr)₂ in benzene-d₆ exhibits six paramagnetically shifted ligand resonances ranging from +195 to -31 ppm that are consistent with a mononuclear high spin (S = 2) iron(II) complex in solution. Upon exposure to air, Fe(L^ipr)₂ forms the oxo-bridged dinuclear iron(III) complex [(L^ipr)₂Fe]₂O as shown by ¹H NMR spectroscopy.