Spontaneous Reduction of a Low-Spin Fe(III) Complex of a Neutral Pentadentate N₅ Schiff Base Ligand to the Corresponding Fe(II) Species in Acetonitrile

The iron complexes of a designed pentadentate Schiff base ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy3) have been synthesized. The low-spin mononuclear Fe(III) complex [(SBPy3)Fe(DMF)](ClO4)3 (2), though stable in the solid state, is spontaneously reduced to the corresponding Fe(II) species [(SBPy3)Fe(MeCN)]2+ in MeCN. Fe(II) complex [(SBPy3)Fe(MeCN)](BF4)2 (3) has been isolated independently and characterized by crystallography. Electrochemical studies indicate that SBPy3, like other pentadentate polypyridine ligands, stabilizes the Fe(II) center to a great extent (E1/2 = 1.01 V vs SCE in MeCN). This fact is responsible for the ready reduction of 2. It is evident that such reactivity has brought complications in the syntheses of iron complexes of polypyridine ligands reported in previous accounts. Very low solubility of 2 in MeOH has allowed isolation of analytically pure 2 in the present work. Storage of dilute methanolic solution of 2 results in the formation of the μ-oxo Fe(III) dimer [(SBPy3)FeOFe(SBPy3)](ClO4)4 (5), the structure of which has also been determined. Fe(II) complex 3 reacts with CN- to afford cyanide adduct [(SBPy3)Fe(CN)](BF4) (4) but does not exhibit any reactivity toward NO. The azomethine moiety (CHNpy) of 2 is rapidly oxidized by H2O2 to a pyridine-2-carboxamido (C(O)Npy) unit and affords [(PaPy3)Fe(MeCN)](ClO4)2 (1), a complex previously reported by us.